Iron deficiency is the most common micronutrient deficiency in the world, affecting 1.3 billion people (24% of the world's population). Severe iron deficiency, i.e., iron deficiency anemia, is particularly debilitating, since iron has several vital physiological functions, including: (1) carriage of oxygen from lung to tissues; (2) electron transport within cells; and (3) participation as a co-factor of essential enzymatic reactions in neurotransmission, synthesis of steroid hormones, synthesis of bile salts, and detoxification processes in the liver. Among the consequences of iron deficiency anemia are an increase maternal & fetal mortality, an increased risk of premature delivery and low birth weight, learning disabilities & delayed psychomotor development, reduced work capacity, impaired immunity (high risk of infection), an inability to maintain body temperature, and an associated risk of lead poisoning because of pica.
Iron deficiency anemia commonly affects patients having chronic diseases, such as kidney disease, inflammatory bowel disease, cancer, HIV, and diabetes. Iron deficiency also afflicts mammals after blood loss and females after parturition.
It is well known to treat an iron deficiency with orally administered iron supplements. In general, relatively large doses of oral iron fortificants are needed to achieve a desired therapeutic effect. The absorption of non-heme iron from the gastrointestinal tract varies from 2% to greater than 90% because it is strongly influenced by the iron status of the body, the solubility of the iron salts in aqueous solutions, the integrity of gut mucosa, and the presence of absorption inhibitors or facilitators in ingesta. For example, foods which contain polyphenol compounds and/or phytic acid bind with dietary iron, decreasing the concentration of free iron in the gut and forming complexes that are not absorbed. Cereals such as wheat, rice, maize, barley, sorghum and oats; vegetables such as spinach and spices; legumes such as soya beans, black beans, and peas; and beverages such as tea, coffee, cocoa and wine contain substances that inhibit iron absorption from the gut. Likewise, L-ascorbate and L-cysteine are known to facilitate absorption of ferrous iron.
Oral administration of iron supplements is known to be commonly accompanied by undesirable side effects, including nausea, vomiting, gastric irritation, constipation, and black stools. For these and other reasons, patient noncompliance with dosage regimens is also a common problem. In addition, conventional iron fortificants present safety concerns, since intolerance to conventional iron salts and accidental overdosing of iron is one of the leading causes of hospitalization in adults and children and is a leading cause of death children under the age of six.
Ferric Citrate. Ferric citrate (Chemical Abstracts Registry No. 12338-05-8) is an iron(III) citrate salt composed of ferric iron and citrate ions in an undefined molecular composition. Ferric citrate is used as an iron fortificant and hematinic agent. [The Merck Index, 14th Ed., M. J. O'Neil, P. E. Heckelman, C. B. Koch, K. J. Roman, Eds. Merck & Co., Inc., Whitehouse Station, N.J., 2006, Monograph No. 4021, page 687.]
When freshly prepared by reaction of an iron(III) salt and citric acid in solutions having a low pH, ferric citrate is a 1:1 iron:citrate complex that is green in color and has a weight percent composition of about 17.9% iron(III) and 62.2% citrate. (The other 20 weight percent of the composition consists of water and other anions and amines derived from the iron salt.) [Structure and Bonding, Volume 6. P. Hemmerich, C. K. Jørgensen, J. B. Neilands, R. S. Nyholm, D. Reinen, and R. J. P. Williams, Eds. Springer-Verlag, New York, pages 132-134.] Ferric chloride, ferric sulfate, and ferric nitrite are examples of iron(III) salts having a low pH, and U.S. Pat. No. 6,903,235 confirms in its disclosures that a pharmaceutical-grade ferric citrate having a weight percent composition of about 17.9% iron(III) and 62.2% citrate (a 1:1 complex of iron and citrate) is formed by reacting ferric chloride, ferric sulfate, or ferric nitrite with citric acid in alcohol solution.
During storage the chemical composition of the ferric citrate complex does not change. However, even when protected from light, the physical characteristics of ferric citrate change significantly. For example, the color of ferric citrate changes from green to garnet red or pale brown. In addition, the solubility of the complex changes from water soluble to water insoluble. It has long been known that these and other physical changes mark a change in composition from low molecular weight complexes having a formula weight of less than about 1,000 Daltons to high molecular weight polymers having formula weights estimated at greater than 10,000 Daltons. [Structure and Bonding, Volume 6. P. Hemmerich, C. K. Jørgensen, J. B. Neilands, R. S. Nyholm, D. Reinen, and R. J. P. Williams, Eds. Springer-Verlag, New York, 1969, pages 132-134.] Ferric citrate polymers are spherical in shape when examined by electron microscopy, and the iron ions are tightly bound to bridging oxygen atoms, not citrate, and buried deep within the sphere. Citrate is believed to be located at the surface of the spheres, where it acts to stabilize the surface of the oxy-iron polymer, prevent additional cross-linking and polymerization to chemical entities having molecular weights that greatly exceed 10,000 Daltons, and prevent precipitation of the polymers formed thereby at high pH.
Polymerization of ferric citrate also adversely changes the biological properties and bioavailability of the ferric citrate complex. In fact, the biological properties and bioavailability of polymeric ferric citrate more closely resemble those of ferritin, the physiological storage form of iron, than physiologically bioavailable iron, as in iron salts such as ferrous sulfate. For example, only a small percentage iron from polymeric ferric citrate is available to cross membranes. [Structure and Bonding, Volume 6. P. Hemmerich, C. K. Jørgensen, J. B. Neilands, R. S. Nyholm, D. Reinen, and R. J. P. Williams, Eds. Springer-Verlag, New York, 1969, pages 132-134.] Likewise, Gebran et al. have reported that iron polymers significantly impair the immune response in mammals. [S. J. Gebran, E. L. Romano, A. Soyano. Iron polymers impair the function and maturation of macrophages. Immunopharmacol Immunotoxicol 1993; 15(4): 397-414.] For example, Gebran et al. reported that the phagocytic capacity of ferric citrate-treated macrophages was inhibited in a dose-related manner. Cell viability was not affected, although the level of lipid peroxidation, an undesirable change to the lipids and lipid membrane, was significantly elevated as the result of exposure. Further, the capacity of low density, nonadherent bone marrow cells to form colonies of macrophages was also inhibited significantly by polymerized ferric citrate. In a second study, this research group found that polymerized ferric citrate suppressed the PHA-induced proliferative response and E. rosette formation of human lymphocytes in culture. [(a) A. Soyano, H. Pons, R. Montano, E. Roman, A. Muller-Soyano, R. Somoza. Effect of iron compounds on the immune response in vitro. Recent Adv Pharm Therapy 1989; p. 401; (b) A. Soyano, E. Fernandez, E. Romano. Suppressive effect of iron on the in vitro lymphocyte function: Formation of iron polymers as a possible explanation. Int Arch Allergy Appl Immunol 1985; 76:376; (c) A. Soyano, H. Pons, E. L. Romano. Interaction of iron polymers with blood mononuclear cells and its detection with the Prussian blue reaction. Immunopharmacol 1992; 23: 29.]
Once formed, high molecular weight ferric citrate polymers are very slow to dissociate to low molecular weight chemical entities that provide iron. It is known that the bioavailability of iron from ferric citrate polymers is very low. For example, Bates et al. showed that ferric iron from ferric citrate is transferred to transferrin, the physiological iron transporter, over a matter of days. [G. W. Bates, C. Billups, P. Saltman. The kinetics and mechanism is iron(III) exchange between chelates and transferrin. I. The complexes of citrate and nitrilotriacetic acid. J Biol Chem 1967; 242(12): 2810-2815.] It would be desirable to provide an iron complex that would transfer iron within minutes or hours.
It is known that polymerization of freshly prepared ferric citrate can be prevented or slowly reversed under certain conditions. For example, if a 20-fold excess of citrate is present in a solution of freshly prepared ferric citrate, polymerization of the ferric citrate will be suppressed. Likewise, a 20-fold excess of citrate in solution will reverse polymerization over a period of many days. Polymerization is also reversed over a period of hours to days by the addition of amino-acetate iron-chelating agents such as NTA or EDTA. [Structure and Bonding, Volume 6. P. Hemmerich, C. K. Jørgensen, J. B. Neilands, R. S. Nyholm, D. Reinen, and R. J. P. Williams, Eds. Springer-Verlag, New York, 1969, pages 132-134.]
Since the absorption of non-heme iron from the gastrointestinal tract (i.e., its oral bioavailability) is strongly influenced by the solubility of the iron salts in water and the availability of the iron for interaction with physiological iron receptors, such as the divalent metal transporters on enterocytes, and physiological iron transporters, such as transferrin, there is a long-felt need for a stable, bioavailable, unpolymerized form of ferric citrate that is utilized physiologically after oral or parenteral administration. The present invention provides an iron preparation having those properties.